Within recent years, the method of constructing dams using the roller compacted concrete (RCC) technique has become more and more popular. This type of construction is quickly replacing earth or rock filled dams, primarily because of the relatively low initial cost, but also because of the more efficient construction that substantially eliminates leaking, and thus provides the resultant lower maintenance costs. Indeed, with the latest construction techniques being used, the RCC dam can compete on some sites with the more traditional poured mass concrete dams.
The standard of the RCC dam technique utilizes a plurality of precast concrete panels that are erected to form a generally vertical wall facing upstream. A liner is provided on the downstream side of the precast panels and a poured concrete curtain wall is placed against the liner. Behind the curtain wall a damp gravel fill is blended with cement and applied in layers. Each layer is compacted by rolling to complete the construction.
This basic RCC dam construction and method is disclosed and claimed in prior U.S. Patent to Sexton et al. U.S. Pat. No. 4,659,252. The actual implementation of this construction has proven to be highly successful over the years. Not only is the cost of construction compatible with other ways of constructing dams, either earth or rock filled dams or poured mass concrete dams, the RCC dam construction has proven to have the best record in terms of controlling continuing maintenance cost.
However, one key problem that has arisen that was not originally anticipated is concerned with the attachment of the plastic liner to the face of the precast concrete panels. Originally, raised T-shaped bars were included on the backside of the plastic liner for embedding into the cast concrete panel. While this approach has proven successful, one potential shortcoming has been identified. After erection of the dam, if there is a slight shift in the foundation due to unexpected settling and/or an earth tremor occurs in the area, the T-shaped bars are not as forgiving as is desirable. In effect, the bars secure the liner to the precast panels in a semi-rigid fashion, so that when a shift occurs undesirable stress concentrations can occur in the liner and/or the elongated sealing strips. The undue stressing of the liner and the sealing strip can eventually lead to weakening of the dam structure and cause an increased maintenance cost, such as represented by a need for resealing of the site involved.
Recently, the application of polymer sheets for outdoor use to form protective barriers and covers of many types have become more and more popular. In broad terms, these sheets have become known as geomembranes, and in particular are used not only for dam structures, as described in the '252 patent, but are also recognized for use in like structures, such as ponds, hazardous waste containment pits, landfills, canals, tunnels, and the like. It is also known to provide the geomembranes with a backing of non-woven or woven synthetic material, known as geotextiles. In its broadest sense, this type of plastic or polymer sheeting with a fabric backing is offering advantages of not only lower cost, but also more efficient performance and ease of installation into the particular containment assembly involved. In its broadest sense, the geomembrane and geotextile form a geocomposite, as is illustrated and described best in U.S. Pat. No. 5,507,900, and the references cited therein, and this patent is incorporated herein by reference.
In my previous U.S. Pat. No. 5,143,480 I have described a dam structure and a method for protection of the dam structure from excess moisture utilizing this geocomposite. In this prior patent, the liner is placed on the upstream side of the dam, such as a poured mass concrete dam, and has overlapping edges for sealing between the individual sheets. The geotextile backing is attached to a mesh semi-open sheet that allows drainage of all moisture, such as condensation, to protect the dam structure.
Similarly, there have been other uses of geomembranes in dam like structures where modules are formed with spacers and attached by flanges or other mechanical fasteners. One example of this type of prior art structure is shown in the Ledeuil U.S. Pat. No. 4,913,583. Still today, as represented by the Lisart U.S. Pat. No. 4,832,528, a significant faction proposes precast panels without the benefit of geomembrane sealing.
Accordingly, while geomembranes and/or the combination with a backing of geotextiles is known in the art of constructing hydraulic structures, and in particular with regard to use in RCC dam construction, to date the application of this technology has been for uses other than the type of use in the standard RCC dam, as shown and described in the '252 patent. It is also clear that the prior proposed uses have not addressed the problem of minimizing the stress in the plastic liner of the dam that can be caused by a slight shifting in the panels after the construction of the dam is finished. Thus, there is a need to provide an improved panel assembly for a dam structure or the like, and an improved method of construction. The panel assembly and method should make the best use of the geomembrane liner. It is contemplated that adding a geotextile sheet backing to form the geocomposite can assist in this purpose. Overall, there should be a dramatic improvement in the sealing of the individual panels, and to do so in a manner and to minimize the stress concentration in the geomembrane during the life of the dam.